We propose to assess the responses of the lung (parenchyma and airways) to constrictor agents. Dogs, and both asthmatic and non-asthmatic human subjects will be studied using various lung volume histories before and after inhalation of various constrictor agents. Measurements of parenchymal (static and quasi-static pressure volume curves, dynamic tissue viscance) and airway (central and peripheral conductance, anatomical dead space) responses and measurements assessing the combined responses of parenchyma and airways (maximal expiratory flows with air and helium-oxygen mixtures, maximal flow static recoil curves) will be made. Our aim is to examine the hyposthesis that the effect of various volume histories on the degree of obstruction depends upon the relative degrees of parenchymal and airway hysteresis. If parenchymal hysteresis exceeds that of the airways, a deep inflation will increase obstruction. If airway hysteresis exceeds that of the parenchyma, a deep inflation will decrease obstruction. If the degrees of hysteresis are equal, there will be no effect of volume history on obstruction. A dominant parenchymal constrictor effect (which some agents appear to have) should lead to worsening obstruction after a deep inflation. We will ultimately hope to assess whether volume history effects on obstruction relate to severity and prognosis in acute asthmatic episodes.